1. Field
Example embodiments relate to a top-emitting N-based light emitting device with an increased light extraction rate including a nano-scale patterned transparent conductive thin film that is prepared by wet-etching and annealing the surface of the transparent conductive thin film without using a mask, and a method of manufacturing the same.
2. Description of Related Art
When a light emitting device is formed using an N-based compound semiconductor (e.g., GaN-based compound semiconductor), a process of forming a high quality ohmic contact between the semiconductor and the electrode is required. However, even though much research on light emitting devices using N-based compound semiconductors has been recently conducted, a high quality ohmic contact cannot easily be obtained because a p-GaN has low carrier concentration, high sheet resistance, and low electrical conductivity. Thus, the high quality ohmic contact of the p-GaN using a metal electrode becomes more essential to provide a good current spreading in the light emitting device. Particularly, a high quality ohmic contact having low specific contact resistance and high light transmittance is required because light generated in a top-emitting light emitting device exits through the ohmic electrode.
A structure in which nickel and gold are laminated on a p-type clad layer is typically used as an ohmic contact structure in top-emitting light emitting devices. The Ni/Au ohmic contact structure annealed in an oxygen or air atmosphere has been known to form a semi-transparent ohmic contact layer with low specific contact resistance of about 10−3 to 10−4 cm2. However, the conventional Ni/Au ohmic contact layer is not suitable for use in large capacity and high brightness light emitting devices due to the low light extraction rate by having a low light transmittance of about 75% when the thickness of the layer is 450 nm. Thus, research involving using a transparent conductive oxide (e.g., ITO) having a better light transmittance than a semi-transparent Ni/Au structure, which is used as a conventional p-ohmic contact layer, has been conducted in order to improve the limited output power of such top-emitting light emitting devices. A top-emitting light emitting device having an improved light output power of about 1.3 times has been developed by employing an Ni/ITO ohmic contact layer which has a light transmittance of 86.6%, which is higher than the light transmittance of the conventional Ni/Au ohmic contact layer of about 71.1% when the thickness of the layer is 450 nm.
Electrode-surface texturing has been recently introduced in an effort to maximize the light extraction rate of light emitting devices. A light emitting device has improved light output power of about 16% by forming a p-type GaN ohmic contact layer using an NiO/ITO structure, patterning an ITO electrode having micro-scale holes using conventional photolithography and dry etching, and employing the patterned ITO electrode to a light emitting device. In addition, a method of forming one dimensional stripe-shaped patterns through interferometric exposure and electron beam exposure using He—Cd laser, Ar laser, or etc. has been developed.
A method of improving external quantum efficiency of a light emitting device by employing a light extraction structure composed of light extraction elements (LEEs), disperser layers, or similar on the surface of the light emitting device using dry-etching employing a photoresist mask and reactive ion etching (RIE) has been developed.
However, electrode-surface texturing and patterning require additional processes involving a mask and dry-etching. Thus, these methods are not effective for manufacturing light emitting devices. The electrode-surface may be damaged by the dry-etching process in the manufacturing of the N-based light emitting devices. Further, the light extraction rate of light emitting devices that emit light having a wavelength in the range of 400 to 500 nm cannot be maximized because the light emitting devices manufactured by these electrode-surface texturing and patterning methods have micro-meter sized hole patterns.